Parallel analog-to-digital converter having random/pseudo-random conversion sequencing

Abstract

In a parallel Analog-to-Digital Converter (ADC) device a number of ADCS work in parallel, the conversion processes in each ADC overlapping the processes in the other ADCs. The number of ADCs and the sampling period at which samples arc taken and new conversion processes are periodically started in the ADCs are selected so that at each instant, at least one ADC is idling not performing any conversion. After the conversion is made by one of the ADCs, a choice is made whether the next sampled value is to be converted by this ADC or by the idling ADC. This choice can be made in a random or a pseudo-random way. Undesired tones existing in the composite output signal of parallel ADC devices having no such extra ADC are transferred to noise, as the error in the output signal caused by differences in the conversion characteristics of the ADCs is distributed in the frequency domain.

What is claimed is:  
     
       1. A parallel converter device comprising: 
       an input terminal for receiving an analog signal;  
       an output terminal;  
       a predetermined number of element converter devices connected to the input terminal and working in parallel for determining digital values from analog values successively sampled, at times periodically repeated with a predetermined sampling period, from an analog signal received on the input terminal, each element device being arranged to provide, after a conversion time period smaller or equal to the sampling period, a digital value representing a sampled analog value to the output terminal, the predetermined number of element converter devices and the sampling period being selected such that at any given moment at least one of the element converter devices is idling and not determining a digital value from an analog value;  
       a selector connected to the element converter devices for selecting, after a conversion period of an element converter device, between one of said element converter devices and the at least one converter device which is idling, to determine a digital value from a next sampled analog value; and  
       a choice generator connected to the selector, the choice generator providing one of a random and a pseudo-random control signal to enable the selector to randomly or pseudo-randomly select between one of said element converter devices that has finished determining a digital value in a previous conversion period and the at least one converter device which is idling, to determine a digital value from a next sampled analog value.  
     
     
       2. A method of converting an analog signal to a sequence of digital values, the method comprising the steps of: 
       sampling, at times periodically repeated within a predetermined sampling period, the analog signal to provide sampled analog values;  
       determining for each analog value a digital value, the determination being made in one of a predetermined number of independently working parallel processes, each determination requiring a predetermined conversion time period smaller than or equal to the sampling period;  
       combining the determined digital values to form a sequence; and  
       choosing, at the instant when a next analog value is sampled, which one of at least two of the parallel processes is to start determining a digital value for the next analog value in one of a random and a pseudo-random manner, the at least one non-chosen parallel process idling and not making any determination of digital values until an analog value is again sampled;  
       wherein the choosing is made between one of the parallel processes that has finished determining a digital value in a sampling period before said instant and at least one of the parallel processes that has not made any determination of digital values during the sampling period before said instant.